Surely reinforced Dreugh-coral strong enough to support submarine citadels is no more outrageous than making armor from bonemeal-plaster or from volcanic glass? Note that calcinium is not chalk, but forms the basis for chalk deposits as a form of calcite. The armor/weapon reference is to address that it was a material in ESO workable into armor/weapons, but isn't really a practice done too often. If chalk was a suitable replacement for calcinium, surely all the chalk cliffs would have been mined away long ago.

[The following is a fragment from the the Imperial Flora and Fauna Survey's "Guide to Wild and Settled Dreugh", focusing on the structures and materials of the Cephalomer and their less settled counterparts.] Ancient architecture of the Dreugh is best exemplified by calcinium ("calcareous-city" in Aldmeris), a word that refers to both the submarine colonies themselves and the calcareous, coral-like secreted material that form the base of their structures. Calcinium plays an important role for the biology and society of all forms of the Dreugh life cycle, though much of what we know its use is limited to that of those who have undergone karvinasim. Information on underwater society is limited to a small range of sources without much in the way of verification. The Cephalomer traditionally lead isolated lives, and take great offense to the threat of an outsider encroaching their territory and tainting the 'waters of Bal'. Further reports of lurking beasts such as the massive Dreugh "Man-O-War" hinder field research on the upper echelons of Dreugh society. Still, there are few and sometimes fanciful accounts of the underwater citadels of the Dreugh, including the most famous description of the Trident-Kings' "cities of glass and coral" in the the Thirty-Six Lessons of Vivec. It can be deduced that this 'coral' is in fact calcinium secreted by Cephalomer, and not that of the undead reefs that the Sload shape with necromantic practices. In Morrowind, the coasts of the Boethian Peninsula are characterized by large, unbroken cliffs of chalk. It appears that this impressive land feature is formed from ancient deposits of calcinium from the Padomaic Ocean, making it the largest known concentration of calcinium outside of a Dreugh citadel, and serves as evidence for some of the oldest known Dreugh structures of the kalpa. As a material, calcinium finds use in alchemy for its magical properties and in structures for its durability in and ability to retain water. Though one cannot loot the material from or even approach the walls of great Dreugh cities underwater and expect to live, traces of calcinium are typically left behind in the mud-reinforced nests and prey-cocoons of land dreughs. During Pasdali, Certain Moramori-Cyrod tribes gather to collect calcinium and grom from hundreds of such abandoned nests to form sacred masks and armor, though the tradition is largely obscured in modern times. The Imperial Archeological Society has uncovered samples of armor and even weapons fashioned from calcinium and newtscale dating back to the middle Interregnum and earlier, though it is not clear when the tradition started. The ancient design of calcinium armor is able to withstand high-pressure environments deep in the Topal Sea, and when linked carefully around a ril of newtscale, does little to supress mobility underwater. The calcinium finds further use on the poles of spears and other such weapons to provide more efficient usage and durability in underwater hunting and fishing.